Nitrogen Pressure Swing Adsorption (N.sub.2 PSA) is a separation process commonly used for the production of nitrogen. The feed to this process is typically air but may be nitrogen enriched air. It is typically a two bed process operating with a relatively simple cycle that includes pressurization, product draw at high pressure, pressure equalization of the beds and depressurization. Other steps such as product purge may be used. The total cycle time is on the order of minutes. The high pressure is typically 4 to 8 atmospheres and depressurization is typically to atmospheric pressure though vacuum could be employed.
N.sub.2 PSA is a kinetics-based process, i.e.; the separation of O.sub.2 and N.sub.2 occurs because of a kinetic rather than an equilibrium selectivity of the adsorbent. During the high pressure feed step, O.sub.2 is selectivity adsorbed and an N.sub.2 enriched product is withdrawn. During the low pressure blowdown step, the O.sub.2 -rich adsorbed phase is desorbed and removed from the bed. The adsorbent commonly employed in commercial units is carbon molecular sieve (CMS).
Carbon molecular sieves are usually prepared by treating a carbonaceous material (for example: coal, coconut shell char, peat, pitch, carbonized polymers, and the like) with additional carbon-containing species. U.S. Pat. No. 3,801,513 Munzner, et al., (1974) describes obtaining carbon molecular sieves (CMS) for oxygen separation by treating coke having volatile components of up to 5% with a carbonaceous substance that splits off carbon at 600.degree. to 900.degree. C., thereby narrowing the pores present in the coke. The starting coke can be derived from coal, peat, coconut shell, wood or plastics. It has been stated by others that the average pore size of the adsorbent must be below 3 angstroms to effect oxygen separation from nitrogen. The average pore diameter can be adjusted by changing the intensity of the treatment. Example 6 of U.S. Pat. No. 3,801,513 describes coconut shell material having a particle size of 1 to 3 mm which is heated at 3.degree. C. per minute to 750.degree. C., the volatiles being equal to 4.5%, where it is held for 30 minutes while ethylene gas is introduced during the holding period. The material is then cooled under nitrogen. In an evaluation test, a gas product was reported containing 49.5% nitrogen and 50.5% oxygen.
At about the same time, attention was directed to the use of other materials as the base material for making carbon molecular sieves. Japanese Publication No. Sho 49-37036 (1974) describes making a carbon molecular sieve by condensing or polymerizing a phenol resin or furan resin, so that the resin is adsorbed on a carbon adsorbent and thereafter carbonizing the product by heating. The carbonizing can be carried out at 400.degree. to 1,000.degree. C. in an inert gas. The heating rate given is 50.degree. to 400.degree. C. per hour (0.8.degree. to 6.7.degree. C. per minute) and an example is given of heating at 6.7.degree. C. per minute to 650.degree. and 800.degree. C. where the material is held for 1.5 hours. The operation is said to reduce the pore diameter of the carbon adsorbent.
Coconut shell char is a commodity material readily available commercially and is often cited as a suitable base material for preparation of carbon molecular sieves by various modifications. Very little description has been provided, however, about the preparation of the coconut shell char itself. Shi Yinrui, et al., "Carbonization of Coconut Shells", Forest Products Chemistry and Industry Institute, Chinese Academy of Forestry, Vol. 6, No. 2, pages 23-28 (1982), describes making coconut char for the production of activated carbon by heating the shells to 720.degree. C. Carbonization is said to be complete at 550.degree. C., using heating rates of 10.degree. and 20.degree. C. per minute. At lower carbonization temperatures, it is stated that the rate should be less than 10.degree. C. per minute.
U.S. Pat. No. 4,594,163, Sutt, Jr., (1986) describes a continuous process for making a CMS beginning with a charred naturally occurring substrate and using non-activation conditions, i.e. non-oxidizing, and no addition of pore-constricting or blocking materials. The process is stated to involve heating the char, e.g., coconut shell char made by the process described in U.S. Pat. No. 3,884,830, to 900.degree. to 2,000.degree. F. for 5 to 90 minutes. The examples heat to 1800.degree. F. (982.degree. C.) and higher. The CMS product is said to have improved oxygen capacity at 25.degree. C. of 4.00 to 6.00 cc/cc, and average effective pore diameters of 3 to 5 angstroms. As an example, representing prior art, charred coconut shell is prepared by heating at 5.degree. C. per minute to 500.degree. C., crushing and sieving the char to obtain 20.times.40 mesh (U.S. sieve) material and then treating in nitrogen by heating at 5.degree. C. per minute to 950.degree. C. and holding for 2 hours. The product had a volumetric oxygen capacity of only 0.8 cc/cc. For the preparation of the coconut shell char, reference was made to U.S. Pat. No. 3,884,830, Grant (1975), which describes preparing activated carbon from starting material such as bituminous coal and charred materials such as coconut char. The coal or char is crushed, sized and mixed with a binder and either agglomerated or compressed into shapes which are then crushed and screened. Activation proceeds by air baking at 300.degree. to 400.degree. C. and calcination at 850.degree. to 960.degree. C. No information is given on preparation of the starting charred materials.
U.S. Pat. No. 4,627,857, Sutt, Jr., (1986) describes preparing a CMS for oxygen/nitrogen separation by continuous calcination of agglomerated non-coking or decoked carbonaceous material, such as coconut char. The agglomerated substrate includes a thermal binder and is sized and screened or pelletized. Calcining is carried out under inert gas purge at 250.degree. to 1,100.degree. C. for at least one minute, preferably 10 to 60 minutes. Examples give oxygen capacities at 25.degree. C. for the product CMS of 2.25 to 4.44 cc/cc. For information on the starting char material, reference is made to the above mentioned U.S. Pat. No. 3,884,830.
U.S. Pat. No. 4,629,476, Sutt, Jr., (1986) describes making a CMS said to have improved selectivity for gas or liquid separations by impregnating a carbonaceous substrate, e.g. coconut shell char, with an organic polymer having a molecular weight of at least 400 or an inorganic polymer at a dosage rate of at least 0.001 wt. %. Further modification of the impregnated sieve by charring at 250.degree. to 1,100.degree. C. is disclosed.
It is common in duscussions of preparing CMS from coconut shell charcoal to direct the preparation of the char into a pellet for use in separation processes. U.S. Pat. No. 4,742,040, Ohsaki, et al., (1988) describes making CMS by combining coconut shell charcoal with a binder of coal tar or coal tar pitch, pelletizing and carbonizing the pellets at 600.degree. to 900.degree. C., removing soluble ingredients from the pellets with a mineral acid, drying the pellets, adding a distilled creosote fraction and reheating to 600.degree. to 900.degree. C. for 10 to 60 minutes. Oxygen capacities at 25.degree. C. of about 6.0 to 7.0 milliliters per gram are disclosed for the product CMS and 8.0 milliliters per gram for the raw carbonized charcoal which is non-selective. A similar approach of converting the carbonized material into pellets is given by U.S. Pat. No. 4,933,314, Marumo, et. al. (1990) which describes making CMS from spherical phenol resin powder mixed with a binder, pelletized and heated to carbonize the pellets. In making the CMS, various materials such as finely divided cellulose, coconut shell, coal, tar, pitch or other resins can be added in small amounts to improve workability, e.g. in pellet molding. The use of pelleted CMS, besides involving expensive processes for forming the pelletized material, invariably suffers from residual binder material or its decomposition products in the pores of the CMS, thereby reducing its overall capacity. It is highly desirable, therefore, to be able to develop a carbon molecular sieve which is granular and can be used directly in an adsorbent bed for separations without going through a pelletizing process.
Modifications of carbonized materials are described to involve various steps other than the deposition of carbonaceous materials to narrow sieve pore openings. For example, Wigmans, "Industrial Aspects of Production and Use of Activated Carbons", Carbon, Volume 27, 1, pages 13-22 (1989) describes activation of carbonized residues of coal, wood, coconut shell and the like using agents such as steam, carbon dioxide and air to expose internal porosity. Above 800.degree. C., oxygen reacts 100 times faster with carbon than do steam or carbon dioxide, so that activation is possible only under mass-transfer-limiting and product-inhibiting conditions. Pore volume and pore enlargement occurs with increasing burnoff, but an optimum in surface area and micropore volume is observed. Temperatures of 800.degree. to 850.degree. C. are said to seem to be optimum without notable pore shrinking behavior.
The value of using carbon molecular sieves for air separation in pressure swing adsorption (PSA) is documented in Seemann, et. al., "Modeling of a Pressure-Swing Adsorption Process for Oxygen Enrichment with Carbon Molecular Sieve", Chem. Eng. Technol., 11, pages 341-351 (1988). This article discusses PSA cycles for separating oxygen from nitrogen and argon using a CMS (commercial CMS N2 material manufactured by Bergwerksverband GmbH, Essen), for which structural data are given as are adsorption equilibria of oxygen, nitrogen and argon at 30.degree. C. It is pointed out that at equilibrium these gases are adsorbed in similar amounts, but oxygen is adsorbed considerably faster because its effective diffusion coefficient is more than 8 times those of nitrogen and argon. Consequently, an almost oxygen-free nitrogen-argon mixture can be recovered during adsorption, and on depressurization of the adsorbent bed, a gas containing over 50 volume percent oxygen may be obtained.
The use of zeolites as kinetics-based adsorbents for this application has been suggested in the literature (D. W. Breck, J. Chem. Education, 41, 678,1964; E. J. Pan et. al. in "New Directions for Sorption Technology", G. E. Keller and R. T. Yang (ed.), Butterworth, 1973; H. S. Shin and K. S. Knaebel, AlChE J., 33,654, 1987; H. S. Shin and K. S. Knaebel, AlChE J., 34, 1409, 1988). While it is recognized that the equilibrium selectivity of zeolites for N.sub.2 over O.sub.2 generally detracts from the kinetic selectivity for O.sub.2 over N.sub.2, the cited references show that a kinetics-based separation process using zeolites is possible. The emphasis of the work reported in these references was on process variables and cycle development. The zeolites employed were small pore materials with oxygen capacities of about 5cc/cc (at one atmosphere and ambient temperature).
The cost of N.sub.2 produced by N.sub.2 PSA is a function of the process productivity (SCFH N.sub.2 product/cu ft of adsorbent) and air recovery (moles of N.sub.2 produced/mole air feed). The recovery and productivity of a unit can be varied by changing the process cycle time and/or other operating conditions. Except in undesirable operating regions, however, increasing one parameter usually results in a decrease in the other.
Since N.sub.2 PSA is a kinetics based process, it was anticipated that the most significant process improvements would be obtained through improvements to the kinetic uptake rates and/or kinetic selectivity of the adsorbent. Generally, faster uptake rates yield higher productivity; and higher selectivity results in higher recovery. It is very difficult to increase both the uptake rates and selectivity of an adsorbent simultaneously, or even to increase one while holding the other constant.
Clearly, the potential for use of kinetic-based adsorbents, particularly carbon molecular sieves, in PSA is very high, but the prior art appears to focus on improving the selectivity and adsorption rates of the adsorbents with little or no attention to enhancing the O.sub.2 capacity of the starting material.